The present invention relates to the field of injection cartridges, and more specifically to injection cartridges of the dual-chamber type. The invention also relates to a method for manufacturing such injection cartridges.
In the field of hypodermic injections, injection cartridges have found a wide use. Such cartridges are provided pre-filled with a liquid composition to be injected and are usually provided with a movable piston-like wall at one end and a wall which can readily be pierced by an injection needle or cannula at the other end. When an injection is to be administered, the user inserts the cartridge into the barrel of a syringe adapted to receive the cartridge, and a liquid connection with the contents of the cartridge is established by an injection needle through the aforementioned piercable wall. By applying pressure on the other, movable wall, the liquid is then forced out through the needle and may be administered to a patient.
Such injection cartridges have a number of important advantages. As they are supplied to the user in a prefilled state, there is no need to fill the syringe by drawing liquid up through the needle from a storage bottle containing the composition to be injected. This of course gives a much improved security against contamination from bacteria and small particles which may be released when the closure of the storage bottle is pierced. Furthermore, by the use of suitable injection devices, the injection from cartridges can be simplified so much that the patient can carry out the injection on himself. This is of great importance, for example to persons suffering from diabetes, who have to take frequent injections of insuling.
A further development of the injection cartridges has been the dual-chamber cartridges. Such cartridges are used when the substance to be injected is not stable in solution, but has to be stored in the form of a dry powder, which is dissolved or dispersed in a liquid phase immediately before administering to a patient. Such cartridges have a movable, piston-like wall at one end and a piercable wall at the other end like those cartridges described in the foregoing, but are additionally provided with a second movable, piston-like wall inside the barrel of the cartridge between its two ends, and also with a bypass channel, which is usually arranged in the wall of the cartridge. Thus, the second movable wall divides the cartridge internally into two chambers one of which containing the dry injection substance and the other containing the liquid phase to be mixed with said dry substance. The liquid phase is usually water or an aqueous liquid. Immediately before administering, the liquid is caused to flow through the bypass channel into the chamber containing the solid substance so that said substance is dissolved or dispersed in the liquid phase. This solution or dispersion may then be injected into a patient. The function of a dual-chamber cartridge will be described in more detail in the following specification.
These dual-chamber injection cartridges have all the advantages mentioned for the single-chamber cartridges above, and in addition provide an added security against the injection substance being degraded when it is in a state of solution or dispersion in a liquid phase. However, they still have a number of shortcomings, which make certain improvements desirable:
The solid substance in the front chamber of a dual-chamber injection cartridge is usually surrounded by a gaseous phase, which is usually air. This means that when the liquid phase is made to flow from the rear chamber through the bypass channel into the front chamber to be mixed with the solid substance, air bubbles will be formed and will adhere to the internal wall of the cartridge. The problem is aggravated by the fact that said internal wall has usually been made hydrophobic to facilitate the movement of the movable walls. The frequency, amount and size of such air bubbles formed are dependent on a number of factors, such as the wetting characteristics between the liquid and the internal wall of the cartridge, the material of the cartridge, and other factors. In all cases, however, the air bubbles formed are very difficult to remove.
The presence of air bubbles in the preparation to be injected is highly undesirable. The most important problem is that if air bubbles above a certain critical size are injected, they may block the capillary blood vessels and give rise to very serious consequences. However, the air bubbles are undesirable even if their size is below the critical value, and even if they are not injected at all. As the air bubbles are compressible, this leads to a decreased accuracy in the dosing of the preparation. Furthermore, due to the magnifying-glass effect of the liquid in the cartridge, the small air bubbles adhering to the internal wall will look much bigger, and this gives rise to anxiety in the user, who is usually aware of the risks associated with the presence of big air bubbles in a preparation for injection.
It is therefore an object of the present invention to provide an injection cartridge of the dual-chamber type wherein the disadvantages mentioned in the foregoing are eliminated. This object is achieved through the present invention.